This invention pertains to a polyacetal composition with improved toughness, wear resistance, and flowability. More specifically, this invention pertains to a polyacetal composition which allows molding of thin-wall parts and can form sliding parts with excellent toughness.
Polyacetal (otherwise known as polyoxymethylene) is a well-known polymer having a good balance of physical properties, such as excellent sliding characteristics, and good flowability in molding. It is frequently used in manufacturing gears, zippers, buckles, cassette hubs, and other mechanical parts.
Various efforts have been made to realize further improvement of the characteristics of polyacetals. For example, as a composition with better impact strength than polyoxymethylene, there is a type of composition prepared by adding thermoplastic polyester elastomer and copolymer of ethylene oxide and propylene oxide to polyoxymethylene (Japanese Kokai Patent Application No. Sho 52[1977]-47046). As a type of polyoxymethylene with high thermal stability and excellent gloss-less surface appearance, there is a composition prepared by adding calcium silicate and a block copolymer of ethylene oxide and propylene oxide into polyoxymethylene (Japanese Kokai Patent Application No. Sho 64[1989]-54053). As a polyacetal composition with excellent dimensional stability after molding, there is a composition prepared from polyacetal copolymer and polyacetal in which one of the terminals of the linear polymers made of a repetition of oxymethylene units is blocked by an alkylene oxide adduct on alcohol (Japanese Kokai Patent Application No. Hei 3[1991]-21657). As a polyoxymethylene composition with improved acid resistance, there is a composition containing polyoxymethylene, polyethylene glycol with a molecular weight of 5000 or smaller, and/or polypropylene glycol and/or polybutylene glycol (Japanese Kokai Patent Application No. Hei 6[1994]-192541). As a polyacetal composition with excellent anti-static characteristics, surface characteristics, and moldability, there is a composition containing polyacetal, polyethylene glycol, and other aliphatic polyethers and phenol-based polymeric compound (Japanese Kokai Patent Application No. Hei 7[1995]-11101). As a composition for which there is no squeaking noise among the moldings made of it when they rub with each other and for which the appearance of the moldings is good, there is a composition made of polyoxymethylene, olefin compound, and polyalkylene glycol (Japanese Kokai Patent Application No. Hei 7[1995]-109403). As a polyoxymethylene composition having good sliding property under high load and at high temperature, there is a composition made of polyoxymethylene, ethylene-.alpha.-olefin copolymer with a melting point in the range of 40-80.degree. C., and tetramethylene glycol-ethylene glycol copolymer or liquid ethylene-.alpha.-olefin random copolymer (Japanese Kokai Patent Application No. Hei 8[1996]-176399). As a composition for molding the hinge parts made of a polyacetal resin with improved hinge characteristics for a practical application, there are the following types of compositions: a polyacetal composition containing an oxyalkylene polymer with molecular weight in the range of 10,000-300,000, for example an ethylene oxide/propylene oxide copolymer (Japanese Kokai Patent Application No. Hei 8[1996]-183115); a composition prepared by adding oxyalkylene polymer with a molecular weight of 1000 or higher and an isocyanate compound or its modified compound into a polyacetal (Japanese Kokai Patent Application No. Hei 8[1996]-259779); and a polyacetal composition prepared by adding a core-shell polymer with a rubber-like polymer as the core and glass-like polymer as the shell and oxyalkylene polymer with a molecular weight in the range of 10,000-300,000 and having 2-8 carbon chains adjacent to each other (Japanese Kokai Patent Application No. Hei 8[1996]-325431).
For a polyacetal, as it has a high crystallinity, and although a high strength can be realized, the toughness is insufficient. Consequently, it becomes brittle in a severe environment such as low temperature, repeated load, etc. This problem is particularly significant for a polyacetal with a low molecular weight and high flowability. For example, for zippers, cassette hubs, chassis parts, buckles, clips, etc., the materials should have good flowability so as to have a good productivity and should have high toughness so as to withstand damage caused by impact. However, there has not been found a polyacetal having such properties.
Consequently, there is a demand for development of a polyacetal composition with excellent wear resistance, good flowability, and improved toughness.
The purpose of this invention is to provide a polyacetal composition which has excellent wear resistance, flowability, and toughness so that it can form moldings having excellent durability in severe environments.
Another purpose of this invention is to provide a polyacetal composition which also has high thermal stability in addition to the aforementioned characteristics.